1. Field of the Invention
The present invention relates to a mercury light source tube such as a fluorescent tube, a light source device having the mercury light source tube, a display device, and a television receiver. The present invention specifically relates to a mercury light source tube suitable for a light source of a display panel such as a liquid crystal display panel, a light source device having the mercury light source tube, a display device, and a television receiver.
2. Description of the Related Art
A display device having a non-self-emissive display panel such as a translucent liquid crystal display panel generally has a light source device provided on a back side of the display panel. As a light source to be incorporated into the light source device, a mercury light source tube such as a fluorescent tube is widely used.
A generally used mercury light source tube includes a tube body made from transparent material such as glass, and electrodes provided at ends of the tube body. Rare gases and mercury are sealed in the tube body, and a thin layer of fluorescent material is formed on an inner surface of the tube body. Electric discharge between the electrodes makes electrons collide against mercury so as to generate ultraviolet light, and the generated ultraviolet light excites the fluorescent material so as to obtain visible light.
During the use of the mercury light source tube, electrons successively collide against the electrodes. When electrons collide against the electrodes, substances that make up the electrodes are dispersed, and dispersed substances and mercury in peripheral portions of the electrodes react with each other and form a mercury compound. Therefore, continuing the use of the mercury light source tube reduces mercury sealed in the tube body, which effectively acts on light emission. The reduction of mercury which effectively acts on light emission decreases an amount of light emitted, which lowers luminance, or the reduction of mercury which effectively acts on light emission stops light emission, which brings the mercury light source tube to the end of its lifetime. In order not to shorten the lifetime of the mercury light source tube, it is preferable to prevent mercury from gathering at the peripheral portions of the electrodes so as to prevent dispersed substances and mercury from reacting with each other.
One of the causes of the gathering of mercury at the peripheral portions of the electrodes is that temperatures in the peripheral portions of the electrodes of the tube body become lower than temperatures in other portions. In general, when a temperature of the tube body is locally lowered, the vapor pressure of mercury is lowered in the low temperature portion. Thus, a force to make the vapor pressure of mercury uniform is exerted in the tube body, and mercury moves to the low temperature portion. Accordingly, when the temperatures in the peripheral portions of the electrodes become lower than the temperatures in the other portions, mercury gathers at the peripheral portions of the electrodes.
Next, the causes of the lowering of the temperatures in the peripheral portions of the electrodes will be described. In general, the mercury light source tube to be incorporated into the light source device or the display device has light source holders made of rubber or resin material attached to the tube body in the peripheral portions of the electrodes. Harnesses arranged to provide electric power to the electrodes are drawn from the electrodes. Therefore, heat radiation through the light source holders or the harnesses may sometimes locally decrease the temperatures in the peripheral portions of the electrodes of the tube body. In addition, if the current is increased during the use of the mercury light source tube, a work function in the electrodes is lowered to decrease an amount of heat generation, so that the temperatures in the peripheral portions of the electrodes may sometimes become lower than the temperature of the gases sealed in the tube body. When the temperatures in the peripheral portions of the electrodes of the tube body become lower than the temperatures in other portions of the tube body, mercury gathers at the peripheral portions of the electrodes for the reasons described above, so that the mercury compound is apt to be generated. As a result, the lifetime of the mercury light source tube becomes short.
In view of the above, mercury is prevented from gathering at the peripheral portions of the electrodes by lowering the temperatures in the portions other than the peripheral portions of the electrodes of the mercury light source tube. For example, O-rings or other radiating members are provided on an outer surface of the tube body in positions at a distance from the electrodes. By such a configuration, the temperature of the tube body is locally lowered in the positions where the O-rings or other radiating members are provided or their peripheral portions, so that mercury gathers at the low temperature portions for the reason described above. As a result, mercury is prevented from gathering at the peripheral portions of the electrodes, and the speed of the reduction of mercury which effectively acts on light emission can be lowered.
However, in the configuration in which the O-rings or other radiating members are provided on the tube body, the number of components is increased, and the number of assembly processes of the light source device or the display device is increased, so that production cost is increased. In addition, if the radiating members are not placed accurately in given positions, mercury gathers at unexpected portions, so that the radiating members contrarily have a detrimental effect on the lifetime of the mercury light source tube. In addition, much expense is required in placing the radiating members accurately in the given positions, and the radiating members could be displaced from the given positions by coming into contact with other constituent members even after being placed.
As a prior art literature relating to the present invention, Japanese Patent Application Unexamined Publication No. Hei10-223182, Japanese Patent Application Unexamined Publication No. Hei11-134930, Japanese Patent Application Unexamined Publication No. 2000-214461, and Japanese Patent Application Unexamined Publication No. 2001-015963 are cited.